1. Field of the Invention
This invention relates to technology for printing an image by forming dots on a print medium.
2. Description of the Related Art
As an output device for images created by a computer, images shot using a digital camera, or the like, printing apparatus that print images by forming dots on a print medium are widely used. These printing apparatus print images by forming dots on a print medium by driving the heads at a suitable timing while running the dot forming head back and forth over the print medium. Also, among the printing apparatus, there are items that form dots only during the forward scan of the dot forming head, but if dots are formed by driving the head during the backward scan in addition to the forward scan, it is possible to print images rapidly. In this way, a method of printing by forming dots during forward scan and backward scan is called bidirectional printing.
With a printing apparatus that performs bidirectional printing, when forming during forward scan and when forming during backward scan, it is necessary to make adjustments in advance of the dot forming timing so as not to have displacement occur for the dot formation positions. This is due to the following kind of reason. For example, if forming dots only during forward scan of the head, one reference position near the end part of the back and forth movement is set, and it is possible to have the dot formation start when the head passes through the reference position (or at a specified timing after passing the reference position). To print one image, it is necessary to have the dot forming head go back and forth a plurality of times, but if the dot formation starts from the same position each time during the forward scan, there is no displacement of the dot position even when forming with dividing of the back and forth movement into a plurality of times.
In comparison to this, when forming dots during the backward scan as well, so that the formed dots start being formed from exactly the end position of the dot line formed during forward scan, it is necessary to suitably adjust the timing of starting formation of dots during the backward scan each time. Of course, so that dots are formed from exactly the end of the dot row formed during forward scan, even if the timing of starting dot formation during the backward scan is adjusted, if there was a tiny difference in the head movement speed during the forward scan and the backward scan, there will be position misalignment between the dots formed during forward scan and the dots formed during backward scan. Because of this, when performing bidirectional printing, the demand for precision for the mechanism that moves the head back and forth becomes strict. Then, when sufficient precision cannot be secured, it becomes necessary to adjust the timing of starting dot formation during backward scan so that displacements of the dot positions do not show up easily. From this kind of reason, for the printing apparatus that perform bidirectional printing, there have been proposed various methods for the adjustment method, with incorporation of an exclusive adjustment mechanism for adjusting the relative timing of forming dots during forward scan and backward scan, and adjustment programs (e.g. Unexamined Patent No. 7-81190, Unexamined Patent No. 10-329381, and the like).
However, high precision is demanded for this kind of adjustment, so there is of course the problem that the adjustment mechanism and adjustment program become complex and large. Also, to perform bidirectional printing, because there is demand for high precision for the mechanism for moving the dot forming head, there is the problem that the head movement mechanism also tends to become complex and large. Because of this, even when the dot formation position is slightly displaced, by suppressing to a minimum the effect on image quality, there is demand to develop technology that will make it possible to try to simplify the dot formation position adjustment mechanism and the adjustment program, as well as the head movement mechanism. Furthermore, this kind of problem is caused not just by the displacement in the main scan direction for bidirectional printing, but also due to, for example, shifts in the dot formation position due to physical reasons including mechanical errors such as Sub-scan direction displacement or time errors such as displacement of the ink spray timing. Furthermore, it occurs not just due to dot formation position misalignment, but also to the displacement itself of the timing for forming the dots.